Stripping is a process used to remove volatile compounds from water. The basic concept is to bring the contaminated water into intimate contact with a stripping gas, frequently air, so that the volatile compounds undergo a phase change from liquid to vapor and are carried away by the stripping gas. A number of interrelated design factors affect the stripping efficiency: the Henry's law coefficient, the stripping gas:water volume flow ratio, the contact time and the mass transfer rate. The gas:water volume ratio used to remove organic compounds from water depents on the volatility of the compound to be removed, its concentration in the feed water and the physical attributes under which the contact is carried out. It is typically in the range 50:1-500:1 or more. The organic compound is, therefore, diluted by this amount when it is transferred from the water to the gas. When other factors are constant, a high gas:water volume ratio provides a high percentage of organic compound removal from the water, but creates large volumes of gas contaminated with dilute concentrations of organic compound. A low gas:water volume ratio may provide insufficient dilution of the organic compound in the gas to maintain a good driving force for mass transfer. Under optimum conditions, transfer of the organic compound from the water to the gas can be very efficient and removal rates up to 99.99% can be achieved.
The principal disadvantage of gas stripping is the air pollution that is caused when the waste gas is discharged. Various treatments have been proposed for this exhaust stripping gas. U.S. Pat. No. 4,892,664 described an air-stripping system followed by catalytic oxidation of the contaminated air. U.S. Pat. No. 4,857,198 describes air-stripping in combination with a mixed carbon adsorption/biological treatment for the waste air. U.S. Pat. No. 4,517,094 also briefly mentions combinations of air stripping and carbon adsorption.
That membranes have the capability to separate organic or inorganic vapors from other gases is known. For example, U.S. Pat. No. 5,032,148 describes a membrane fractionation process used to divide a gas stream containing organic vapor into a dilute stream containing less than 0.5% vapor and a concentrated stream containing more than 20% the saturation concentration of the organic compound. U.S. Pat. No. 4,906,256 describes the separation of fluorinated hydrocarbon vapors from other gases by means of membranes. U.S. Pat. No. 4,553,983 describes a basic process for recovering organic vapors from air.